1. Field of the Invention
The present invention relates to a development roller and a developer unit employing the development roller, designed for electrophotographic image forming apparatus such as copying machines, printers, and facsimile machines.
2. Discussion of Background
There is a tendency to simplify the maintenance of electrophotographic image forming apparatus such as copying machines, printers, and facsimile machines, in particular, small-sized units. For this purpose, a developer unit for developing latent electrostatic images formed on an image-bearing member such as a photo-conductor, sensitized paper, or recording sheet using a mono-component developer is disclosed in Japanese Laid-Open Patent Application 53-3233. In fact, that kind of developer unit has been put to practical use.
FIG. 5 is a schematic view showing one embodiment of such a conventional development unit. In FIG. 5, reference numeral 101 indicates a photoconductor drum, which comprises a surface layer portion A and a base member B. The surface layer portion A serves as an image-bearing member. In the base member B, which is provided with a function of supporting the image-bearing member A, an electroconductive rubber layer 103 with relatively large elasticity and a flexible metal foil layer 104 such as an aluminum foil are successively attached to a metallic drum 102. The surface layer portion A comprises a photoconductive insulator layer 105, which is formed by, for example, depositing a metal such as selenium on the metal foil layer 104 of the base member B.
A development roller 106 is brought into pressure contact with the surface of the above-mentioned photo-conductor drum 101. The development roller 106 comprises a metallic core roll 107 and an elastic layer 108 which is provided around the metallic core roll 107 and made of a synthetic rubber or urethane foam, designed to have electroconductivity.
This type of conventional developer unit has the problem that a plasticizer and low-molecular-weight material contained in the elastic layer 108 of the development roller 106 may cause bleeding therefrom and contaminate the surface layer portion A of the photo-conductor drum 101. Further, toner particles are undesirably attached to the surface of the elastic layer 108 of the development roller 106 as a result of the phenomena of tacking and filming, As a countermeasure against such problems, it is proposed to coat the elastic layer 108 of the development roller 106 with a surface layer (not shown) comprising a resin that is readily releasable from the toner. The above-mentioned proposal is made to prevent the bleeding of the plasticizer or low-molecular-weight material from the elastic layer of the development roller, and at the same time, to prevent the tacking and filming of toner to the development roller.
A bias source 109 is connected across the core roller 107 of the development roller 106 and the base member B of the photoconductor drum 101. Above the development roller 106, a hopper 111 containing a mono-component nonmagnetic toner 110 is disposed with an opening at the bottom of the hopper 111 being positioned at a predetermined distance from the surface of the development roller 106. A triboelectric charging member 112 is attached to the inner wall on the right side of the hopper 111. A toner layer formed around the development roller 106 is leveled by a leveling member 113 in such a configuration that the surface of the leveling member 113 is brought into pressure contact with the surface of the development roller 106. The leveling member 113, which is prepared by successively attaching a rubber layer 115 and a triboelectric charging layer 116 onto a metallic roll 114, is not rotatably mounted, but fixed.
According to the development process effected by the developer unit of FIG. 5, a thin toner layer retained on the surface of the development roller 106 is brought into contact with the photoconductor drum 101 bearing latent electrostatic images thereon. The toner is transferred to the photoconductor drum 101 in accordance with the development field, so that the latent electrostatic images are developed and made into visible toner images. In the case of the development roller 106 of FIG. 5, the polarity and the charge quantity of the toner 110 are controlled by triboelectric charging, that is, by the contact of the toner with the triboelectric charging member 112 and the contact of the toner with the triboelectric charging layer 116 of the toner supply layer 113. The toner 110 is selectively transferred from the development roller 106 to the photoconductor drum 101 depending upon the polarity of the charged toner and the development field on the photoconductor drum 101. Namely, the toner 110 is selectively deposited on an image portion determined by the latent electrostatic images formed on the photoconductor drum 101. The development unit as shown in FIG. 5 has the advantage that color toners are available for achieving color image formation because a magnetic material is not necessary for preparation of the toner composition.
Japanese Patent Publication 64-1022 discloses another developer unit. This development unit employs a mono-component non-magnetic toner and is designed to feed a development roller with the toner by means of a magnetic brush.
FIG. 6 is a schematic cross sectional view showing one embodiment of the above-mentioned conventional developer unit.
In a developer unit 203 shown in FIG. 6, as a magnetic roller 208 is driven in rotation in the direction indicated by the arrow, a magnetic carrier is magnetically attracted to the magnetic roller 208 so as to charge non-magnetic toner particles. A magnetic brush 212 is thus formed around the magnetic roller 208. A development roller 211 stands between the magnetic roller 208 and an electrostatic-image-bearing member 201. Charged toner particles 207 electrostatically clinging to the magnetic brush 212 are separated therefrom and supported by the development roller 211 in order to develop an electrostatic image formed on the electrostatic-image-bearing member 201. Namely, the non-magnetic toner particles 207 can be shifted from the magnetic brush 212 to the development roller 211 by the action of an alternating field formed between the magnetic roller 208 and the development roller 211. In the developer unit 203, the toner is initially charged by the principle of two-component development, and the charged toner 207 is carried to the development roller 211 by the magnetic brush 212 formed around the magnetic roller 208. Such a development system requires no member coming in contact with the development roller 211, thereby reducing the stress applied to the toner particles 207 deposited on the development roller 211. This makes it possible to secure the developer unit 203 in the image forming apparatus without any replacement.
The conventional developer unit as shown in FIG. 5 produces various problems, as mentioned above. Namely, toner particles are deposited on the surface of the leveling member for regulating the toner thin layer, in particular when the leveling member is in the form of a blade. The toner filming is caused around the development roller. Further, the surface of the development roller readily exhibits stripe-like scratches with time. Furthermore, the toner is caused to deteriorate because of the application of stress thereto, with a result that the charge quantity of toner and the deposition amount of toner on the development roller change with time. In addition, toner deposition on the background takes place at the time of replenishment of toner. In consideration of occurrence of those problems, the developer unit has been necessarily designed as a detachable cartridge that can be replaced after several tens of thousands of prints have been made. To secure the development unit has been found to be impossible.
To solve the above-mentioned problems, the developer unit of FIG. 6 is proposed. However, when the developer unit of FIG. 6 is employed, the photoconductor drum must be charged to a considerably high surface potential, which will shorten the life of the photoconductor drum. Then, some trials are made to decrease the surface potential of the photoconductor drum at the charging step in order to prevent the photoconductor drum from deteriorating. With a decrease in charging potential of the photoconductor drum, the developing bias to be set is necessarily lowered.
Further, in light of the stability in forming dot images, a toner image may be reproduced by a two-valued modulation system, not a multi-valued modulation system. The two-valued system modulates the image pattern using two gradations, that is, presence or absence of a dot, with the density of every dot being the same. According to the multi-valued system, the image pattern is modulated using multiple gradations. However, use of the multi-valued system makes it difficult to obtain high quality image.
In the case where a dot image is reproduced by the two-valued modulation system, a characteristic curve of the print density of a dot image with respect to the electrostatic potential of a latent electrostatic image is required to have a gradient as steep as possible. The above-mentioned gradient will be referred to as xe2x80x9cdevelopment xcex3xe2x80x9d in the present invention. At the same time, it is required to make the difference between the surface potential of an image portion on the photo-conductor and the developing bias as small as possible, The above-mentioned difference will be referred to as a development potential.
Accordingly, it is a first object of the present invention to provide a development roller capable of successfully performing a development step with the development xcex3, ie, the gradient of the above-mentioned characteristic curve being made rather steep, and the amount of toner to be deposited on the image portion of the photoconductor being maximized at the development potential of 150 V or less.
A second object of the present invention is to provide a developer unit employing the above-mentioned development roller.
The first object of the present invention can be achieved by a development roller comprising a core shaft and an external composite layer provided around the core shaft, the external composite layer comprising an elastic layer and a surface layer which are successively overlaid on the core shaft in that order, with the development roller having a volume resistivity of 1.5xc3x97104 xcexa9xc2x7cm or less and an electrostatic capacity of 1.5xc3x9710xe2x88x924 F/m2 or more.
The second object of the present invention can be achieved by a developer unit comprising the above-mentioned development roller which is disposed so as to have a toner deposited on an image portion formed on a surface of a photoconductor, a toner supply roller which is disposed opposite to the development roller and is capable of forming therearound a magnetic brush comprising the toner and a carrier, and brings the magnetic brush into contact with the development roller to supply the toner to the development roller, and a doctor blade disposed in contact with the surface of the toner supply roller to regulate the thickness of a layer of the toner.